diff --git a/AutoDiffHelpers.hpp b/AutoDiffHelpers.hpp
index f48b6e165..9c96e09ef 100644
--- a/AutoDiffHelpers.hpp
+++ b/AutoDiffHelpers.hpp
@@ -286,6 +286,9 @@ superset(const Eigen::Array<Scalar, Eigen::Dynamic, 1>& x,
 
 /// Construct square sparse matrix with the
 /// elements of d on the diagonal.
+AutoDiff::ForwardBlock<double>::M
+spdiag(const AutoDiff::ForwardBlock<double>::V& d);
+
 AutoDiff::ForwardBlock<double>::M
 spdiag(const AutoDiff::ForwardBlock<double>::V& d)
 {